In the start-up phase of v.34 modems, the calling modem is required to identify the type of answer modem, which can be either a v.32 modem, or a v.34 modem. The calling modem performs the identification via detecting the type of answer signal (hereinafter "v.id signal") sent by the answer modem. If the answer modem is a v.32-type, a 2100 Hz signal (hereinafter "ANS") will be sent. In contrast, if the answer modem is a v.34-type, an amplitude-modulated 2100 Hz signal (hereinafter "ANSam") will be sent. For the latter case, the modulating signal is a 15 Hz signal, and the modulation index is 20%.
The task of the v.id detector, therefore, is to determine whether the answer signal comprises an ANS-type signal or an ANSam-type signal. This problem is further complicated by the fact that typically the answer signal is received over a noisy channel. On such channels, the signal to noise ratio may be as low as 6 dB. In such an environment, it has been found that conventional notch-filter type detectors do not work reliably. This is due to the low-frequency (15 Hz compared with 2100 Hz) and low-energy (20% modulation index) nature of the 15-Hz modulating signal of the ANSam signal. This is further due to the high noise on such channels.
As a result, there is a need for an adaptive coherent signal detection method and apparatus suitable for detecting the v.id signal.